DESCRIPTION (from the applicant's description): Despite pharmacological evidence that serotonin is important in pain modulation and opioid analgesia, electrophysiological studies have consistently demonstrated that the activity of presumed serotonergic neurons does not change during either pain behaviors or analgesia. The proposed experiments are designed to provide a better understanding of the physiology and anatomy of serotonergic neurons in the rat pontomedullary raphe magnus (RM) and adjacent nucleus reticularis paragigantocellularis pars alpha (NRPGa) of the anesthetized and unanesthetized rat. The specific experiments proposed are: 1) The first aim of this proposal is to determine an unequivocal physiological marker for pontomedullary serotonergic neurons in anesthetized rats. Using extracellular recording, RM/NRPGa cells will be classified according to criteria that may be useful in identifying serotonergic neurons, such as a long duration action potential, a slow and steady discharge rate, response to midbrain stimulation and sensitivity to high concentrations of a general anesthetic. The physiology of serotonergic cells will then be directly tested by electrophysiologically characterizing neurons, intracellularly labeling them and processing them for serotonin immunocytochemistry. In this way, the applicant hopes to identify the common physiological traits that are shared by serotonergic neurons but are not present in non-serotonergic RM/NRPGa cells. The somatodendritic morphology and axonal projections of physiologically characterized and intracellularly labeled serotonergic cells will be described and compared to that of non- serotonergic neurons in the same region. 2) The physiological characterization of serotonergic RM/NRPGa cells will be extended to the unanesthetized rat. Recordings will be made from pontomedullary serotonergic units in chronically implanted, freely behaving and unrestrained rats. By using criteria established in the first aim, units will be classified as serotonergic or non-serotonergic in the chronically instrumented but anesthetized rat. Anesthesia will then be discontinued and the cell will be recorded and recharacterized in the wake, unanesthetized rat. The influence of behavioral state on unit activity will be studied by recording cell discharge patterns during spontaneous changes in behavioral state. These experiments will determine whether serotonergic neurons and/or non-serotonergic cells in the rat RM/NRPGa alter their discharge rate in accordance with behavioral state. In the final set of proposed experiments, cell activity and the behavioral response to noxious stimuli will be simultaneously recorded during spontaneous changes in behavioral state. A linear regression analysis will be performed to determine whether the discharge rate of serotonergic cells correlates with the withdrawal latency. By using behavioral state as an experimental variable, the influence of serotonergic cell activity upon nociceptive responsiveness can be examined.